Thermoplastic vulcanizates comprise finely-divided rubber particles dispersed within a thermoplastic matrix. These rubber particles are crosslinked to promote elasticity. The dispersed rubber phase is typically referred to as the discontinuous phase, and the thermoplastic phase is typically referred to as the continuous phase.
Thermoplastic vulcanizates may advantageously be prepared by dynamically vulcanizing a rubber with a curative agent while the rubber is being mixed with a thermoplastic resin. Factors which influence the physical properties of a thermoplastic vulcanizate include those related to the physical properties of the rubber discontinuous phase, the physical properties of the thermoplastic continuous phase, the size and the concentration of the particles of the discontinuous phase, the presence of fillers, curatives, and other additives present in the composition, and the like.
Physical properties of thermoplastic vulcanizates are often trade-offs between extremes. A need exists in the art for thermoplastic vulcanizate compositions having balanced properties. Ethylene based elastomers such as ethylene-propylene (alpha-olefin)-diene (EPDM) elastomers are generally polymers of very high molecular weight (as measured by their Mooney viscosity), and are often suitable for use in TPV applications. However, high molecular weight EPDM elastomers inherently possess very high viscosities, e.g., Mooney viscosity greater than 200 ML(1+4@125° C.). This inherent characteristic of EPDM results in difficulties related to the processability of these polymers. Such polymers are not processable when having Mooney viscosities above about 100 ML(1+4@125° C.). To remedy the concerns associated with the high viscosity of high molecular weigh EPDM, extender oil is often added to the polymers to the reactor effluent containing the polymers reduce the apparent viscosity. The presence of extender oil may, however, be a detriment to manufacturing capacity since the extender oil would replace some of the rubber capacity of the plant.
The required level of extender oil depends on the molecular weight of the elastomer, but is usually sufficient to reduce the apparent viscosity of the oil extended EPDM to a Mooney viscosity of about 100 ML(1+4@125° C.) or below. Commercially available very high molecular weight EPDMs, which would be useful in TPVs, typically contain from about 50 to about 125 phr extender oil.
An existing challenge is how to produce an EPDM that is lower in molecular weight and oil extension than current high molecular weight rubbers such as commercial EPDM Vistalon® 3666 (available from ExxonMobil Chemical Co.) without sacrificing performance of the TPV. This much sought goal would improve production capacity in terms of net rubber yield as well as rates that are often limited by the high MW and high oil extension. Polymer structures made with conventional technologies during past efforts have not produced an EPDM that was lower in MW (and oil extension) that is able to match or exceed the properties of unimodal high molecular weight rubbers known in the art.
For example, EPDM as disclosed in WO 00/26296 is directed to a metallocene based ethylene-alpha-olefin elastomeric composition made by a series reactor operation in which the high molecular weight component has a Mooney viscosity not exceeding 120, and is present in an amount no greater than 50 weight percent. Solvent utilized in such process is removed from metallocene based processes utilizing flash evaporation of the solvent, wherein reduced pressure is applied to the reaction product. However, at least a portion of the oil (if present) in the reaction product may become entrained in the solvent being removed under reduced pressure, and may be removed along with the solvent. Metallocene based processes thus do not allow for the introduction of extender oil into the final reaction product until after the solvent has been removed by flash evaporation.
Metallocene based processes may thus be limited to a polymer product having an overall Mooney viscosity of less than about 90 ML (1+4@120° C.) in the absence of extender oil, due to the handling characteristics of such polymers including the difficulties of further processing polymers having a Mooney viscosity above about 90 ML (1+4@120° C.). However, polymer compositions having a Mooney viscosity of less than or equal to about 90 ML (1+4@120° C.) in the absence of extender oil have inferior properties.
WO 2003 066725A2 is directed to bimodal EPDM polymer compositions comprising a major polymer fraction having a Mooney viscosity above 120 ML(1+4@ 125° C.), and a minor polymer fraction having a Mooney viscosity of 120 ML(1+4@ 125° C.) or less, where the composition has a tan delta of 0.5 or less (125° C./10.4 rad/s). These compositions are essentially free of extender oil and preferably have a Mooney viscosity below 100 ML(1+4@125° C.) to ensure ease of processability. The compositions are prepared using metallocene catalysts in a series reactor process wherein the high molecular weight component is produced in the first reactor, and the low molecular weight component is produced in the second reactor, both using metallocene catalysts. Both components have relatively narrow molecular weight distributions with a polydispersity index (Mw/Mn) of less than 4, preferably less than 3. In addition, both components have a relatively high average branching index factor of greater than 0.7, preferably greater than 0.8, on a scale in which a branching index of 1 represents a linear polymer.
Numerous attempts have been made to produce an EPDM that is lower in molecular weight and oil extension as compared to oil-extended unimodal rubbers (e.g., Vistalon® 3666) to improve plant capacity in terms of net rubber yield, as well as rates of production which are limited by the high molecular weight and high oil extension required by rubbers known in the art. Polymer structures made with the conventional technologies could not produce an EPDM that was lower in molecular weight and lower in oil extension and yet match or exceed the properties of unimodal high molecular weight rubbers in TPV formulations.